Image processing device, printing device, printing system, and image processing method

1. An image processing device configured to process an image constituted by a plurality of pixels arranged two-dimensionally, comprising:

a dividing section configured to divide the image into a plurality of blocks;

a first processing section configured to compare image data representing a gray level of the pixel included in a first block as one of the blocks divided into with a first threshold value prepared in advance, and convert the image data into first dot data representing whether to form a dot using an error diffusion method; and

a second processing section configured to perform processing of comparing image data representing a gray level of the pixel included in a second block as a block adjacent to the first block with a second threshold value prepared in advance, and converting the image data into second dot data representing whether to form a dot using an error diffusion method independently of the first processing section, wherein

the first threshold value and the second threshold value in the first and second processing sections include variable components periodically varying along two-dimensional arrangement directions of the pixels, respectively, and the variable components are provided with the same phase at a junction between the first block and the second block.

2. The image processing device according to claim 1 , wherein

the variable components are one of a positive value and a negative value obtained based on one of a sine wave and a cosine wave, and are added to fixed values deciding the first threshold value and the second threshold value, respectively.

3. The image processing device according to claim 2 , wherein

the fixed value is one of a constant value independent of a magnitude of the image data of the pixel and a value decided in accordance with the magnitude of the image data.

4. The image processing device according to claim 1 , wherein

a spatial frequency f of the variable components is no lower than 3/mm.

5. The image processing device according to claim 1 , wherein

the first processing section and the second processing section are respectively incorporated in separate devices which operate independently of each other.

6. A printing device comprising:

the image processing device according to claim 1 ; and

a dot formation section configured to form a dot on a print medium in accordance with the first and second dot data obtained by performing the conversion.

7. A printing system comprising:

a dividing section configured to divide an original image constituted by a plurality of pixels arranged two-dimensionally into a plurality of blocks;

a first processing section configured to compare first image data representing a gray level of the pixel included in a first block as one of the blocks divided into with a first threshold value prepared in advance, and convert the first image data into first dot data representing whether to form a dot using an error diffusion method;

a second processing section configured to perform processing of comparing second image data representing a gray level of the pixel included in a second block as a block adjacent to the first block with a second threshold value prepared in advance, and converting the second image data into second dot data representing whether to form a dot using an error diffusion method; and

a printing section configured to combine the first dot data and the second dot data with each other to print an image corresponding to the original image on a print medium, wherein

the first processing section and the second processing section are respectively disposed in a first device and a second device coupled to each other via a line, and

the first threshold value and the second threshold value in the first and second processing sections include variable components periodically varying along two-dimensional arrangement directions of the pixels, respectively, and the variable components are provided with the same phase at a junction between the first block and the second block.

8. The printing system according to claim 7 , wherein

the first device is a computer configured to handle the original image, and includes the dividing section and the first processing section,

the second device is a printer, and includes the second processing section and the printing section,

the computer further includes a transmission section configured to transmit the second image data and the first dot data to the printer, and

the printer further includes a reception section configured to receive the second image data and the first dot data.

9. An image processing method configured to process an image constituted by a plurality of pixels arranged two-dimensionally, comprising:

dividing the image into a plurality of blocks;

performing first processing of comparing image data representing a gray level of the pixel included in a first block as one of the blocks divided into with a first threshold value prepared in advance, and converting the image data into first dot data representing whether to form a dot using an error diffusion method; and

performing processing of comparing image data representing a gray level of the pixel included in a second block as a block adjacent to the first block with a second threshold value prepared in advance, and converting the image data into second dot data representing whether to form a dot using an error diffusion method independently of the first processing, wherein

the first threshold value and the second threshold value include variable components periodically varying along two-dimensional arrangement directions of the pixels, respectively, and the variable components are provided with the same phase at a junction between the first block and the second block.